Environmental hiogeography of near-surface phytoplankton in the southeast Pacific Ocean

Biogeographic interpretation of large-scale phytoplankton distribution patterns in relation to surface hydrography is essential to understanding pelagic food web dynamics and biogeochemical processes influencing global climate. We examined the abundance and biomass of phytoplankton in relation to physical and chemical parameters in the southeast Pacific Ocean. Samples were collected along longitude 110°W, between 10°N and 60°S during late austral summer. Patterns of taxa abundance and hydrographic variables were interpreted by principal components analysis. Five distinct phytohydrographic regions were identified: (i) a north equatorial region of moderate productivity dominated by small flagellates, low nitrate and low-to-moderate pCO2; (ii) a south equatorial region characterized by high primary productivity dominated by diatoms, high nutrient levels, and relatively high pCO2; (iii) a central gyre region characterized by low productivity dominated by small flagellates, low nitrate, and high pCO2; (iv) a sub-Antarctic region with moderate productivity dominated by coccolithophores, moderate nitrate concentrations, and low pCO2; and (v) an Antarctic region with high productivity dominated by diatoms, very high nitrate, and low pCO2. Productivity and average phytoplankton cell size were positively correlated with nitrate concentration. Total phytoplankton abundance was negatively correlated with pCO2, photosynthetically active radiation, and ultraviolet-B radiation. The interaction between phytoplankton carbon assimilation, atmospheric CO2, and the inhibitory effect of ultraviolet radiation could have implications for the global climate. These data suggest that the effects would be greatest at southern mid-latitudes (40–50°S) where present phytoplankton production and predicted future increases in UV-B are both relatively high.